Bidirectional shift register and image display device using the same

ABSTRACT

A bidirectional shift register outputs pulses from a plurality of cascaded unit register circuits in a shift order which is one of a forward direction and a reverse direction. A λth stage of unit register circuit ( 38 ) has two set terminals connected to respective outputs of (λ−1)th and (λ+1)th stages and two reset terminals connected to respective outputs of (λ+2)th and (λ−2)th stages. The unit register circuit ( 38 ) sets, when a pulse is input to any one of the set terminals, a reference point N 1  to an H level, and, when a pulse is input to any one of the reset terminals, N 1  to an L level. The order of phase change of clock signals is reversed according to the direction of a shift, and whether a start trigger signal is applied to a top stage or a bottom stage is switched.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese application JP2010-141990 filed on Jun. 22, 2010, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a bidirectional shift register whichcan switch the order of output of pulses and an image display devicewhich uses the bidirectional shift register to drive the respectivescanning lines.

2. Description of the Related Art

Higher resolution of a liquid crystal display device is materialized byimproving the arrangement density of pixels in a display portionthereof. As the arrangement density is improved, the arrangement pitchof various kinds of signal lines for supplying signals to pixel circuitsbecomes finer. Gate lines provided correspondingly to scanning lines ofpixels are connected to gate line driving circuits on sides of a displayregion. A gate line driving circuit includes a shift register foroutputting to the respective scanning lines in sequence a voltage whichenables writing data to a pixel circuit. As the resolution becomeshigher, unit register circuits included in the respective stages of theshift register are also required to be reduced in size.

Ordinarily, a voltage is applied to gate lines in a top to bottom orderof an image correspondingly to an order of input of image data in avertical scanning direction. If the shift register may be drivenbidirectionally, input image data may be written to pixel circuits in abottom to top order when the scan line goes from the bottom to the top.This enables change in the direction of a displayed image with amechanism that is simpler than that in a structure in which a framememory for buffering image data or the like is provided and the order ofthe image data is changed thereby.

A shift register used in a gate line driving circuit or the likeincludes cascaded unit register circuits in a plurality of stages.Basically, operation in which the respective unit register circuits inthe respective stages output a pulse once in an order from one end tothe other end of the row of the unit register circuits is performed, theoperation being linked with vertical scanning or the like.

FIG. 14 is a circuit diagram illustrating a basic structure of a unitregister circuit (see Japanese Patent Application Laid-open Nos.2004-157508 and 2009-272037). In a unit register circuit in an n-thstage, an output transistor M1 is connected between an output terminal(GOUT[n]) and a clock signal source CK, while a transistor M2 isconnected between the terminal (GOUT[n]) and a power supply VOFF. FIG.15 is a signal waveform chart illustrating operation of the unitregister circuit illustrated in FIG. 14. When an output pulse GOUT[n−1]in the previous stage is input to the unit register circuit, a node N3(one end of a capacitor C) connected to a gate of M1 is connected to apower supply VON, and a potential of the node N3 is pulled up to a High(H) level which is a potential at which the transistor is turned on.When the node N3 is at the H level, a node N4 is connected to the powersupply VOFF to be set to a Low (L) level which is a potential at whichthe transistor is turned off, thereby turning off the transistor M2. Inthis way, the unit register circuit is in a set state. In this state,when a clock signal CKV (CK) transitions from the L level to the Hlevel, the potential of the node N3 is further pulled up via thecapacitor C connected between the gate and a source of the outputtransistor M1, and the clock signal CKV at the H level is output fromthe terminal GOUT[n].

On the other hand, in the case of transition of the clock signal CKVfrom the H level to the L level, the potential of the node N3 is pulleddown and a voltage at the output terminal GOUT[n] is also pulled down.Here, linked with a rising edge of a clock signal CKB to an (n+1)thstage, a pulse is generated in an output signal GOUT[n+1] in thesubsequent stage, which is input to the unit register circuit in then-th stage. The pulse of GOUT[n+1] pulls down the potential of the nodeN3. This pulls up a potential of the node N4, the transistor M2 isturned on, and the output terminal is connected to the power supplyVOFF. By the operation, a pulse of the output signal GOUT[n] is output.

SUMMARY OF THE INVENTION

In order to materialize bidirectional drive, both a structure used whenthe drive is in a forward direction and a structure used when the driveis in a reverse direction are provided in the unit register circuit, anda switch element for switching the directions is built in the unitregister circuit. However, a bidirectional shift register which adoptssuch a unit register circuit has problems that miniaturization thereofis difficult and that it is not suitable for higher resolution of animage display device.

Further, at the timing at which an output pulse ends in each of thestages, the potential of the node N3 is abruptly pulled down from apotential higher than the H level to the L level, and the transistor M2is turned on. Such operation is liable to cause unstable behavior atthat timing unless timings and waveforms of signals relating to theoperation are controlled accurately. For example, there is a problemthat the transistor M2 may begin to be turned on before the outputtransistor M1 is completely turned off, which causes a through currentto flow from the power supply VON to the power supply VOFF.

The present invention has been made to solve the above-mentionedproblems, and an object of the present invention is to provide abidirectional shift register which may perform stable bidirectionalshift operation with a simple circuit structure and an image displaydevice using the bidirectional shift register.

A bidirectional shift register according to an aspect of the presentinvention includes a shift register portion including unit registercircuits cascaded in m stages, where m is an integer which is equal toor larger than 3, each of the stages being driven in sequence insynchronization with n-phase clock pulses applied according to an orderof the cascade, where n is an integer which is equal to or larger than3. The unit register circuit in a k-th stage, where k is an integerwhich satisfies 1≦k≦m, includes an output circuit for outputting anoutput pulse P_(k) in synchronization with the n-phase clock pulse whichis input to the unit register circuit with a reference point of the unitregister circuit being at a first potential, a set circuit for settingthe potential of the reference point to the first potential when a setsignal is input, and a reset circuit for setting the potential of thereference point to a second potential when a reset signal is input. Theunit register circuit in the k-th stage, where k is an integer whichsatisfies 1≦k≦m, further includes, as input terminals of the set signal,a forward direction set terminal to which the output pulse P_(k−αf),where k>αf, is input and a reverse direction set terminal to which theoutput pulse P_(k+αb), where k≦m−αb, is input, and, as input terminalsof the reset signal, a forward direction reset terminal to which theoutput pulse P_(k+βf), where k≦m−βf, is input and a reverse directionreset terminal to which the output pulse P_(k−βb), where k>βb is input,provided that αf, αb, βf, and βb are natural numbers which satisfy bothαf<βb<n and αb<βf<n.

The bidirectional shift register according to the one aspect of thepresent invention may be structured to further include: a clock signalgenerating portion; and a trigger signal generating portion, in which:the shift register portion outputs the output pulse P_(k) in a shiftorder which is one of a forward direction and a reverse direction. Inthis structure, the clock signal generating portion supplies the n-phaseclock pulses to the respective stages of the shift register portion insequence in the forward direction in forward shift operation of theshift register portion and in the reverse direction in backward shiftoperation. The trigger signal generating portion generates a forwarddirection trigger signal at the start of the forward shift operation andgenerates a reverse direction trigger signal at the start of thebackward shift operation. The forward direction trigger signal is inputto the forward direction set terminal in each of first to αf-th stagesas the set signal. The reverse direction trigger signal is input to thereverse direction set terminal in each of (m−αb)th to m-th stages as theset signal.

According to the present invention, there is provided an image displaydevice including: a plurality of pixel circuits arranged in matrixcorrespondingly to a plurality of scanning lines; a plurality of gatesignal lines provided for the plurality of scanning lines, respectively,for supplying a gate signal for controlling writing of image data to theplurality of pixel circuits; and a gate signal line driving circuit for,using the above-mentioned bidirectional shift register according to thepresent invention, generating the gate signal to each of the pluralityof gate signal lines based on the output pulse which is output from astage correlated with the gate signal line among the plurality of stagesof the shift register portion.

According to the present invention, there may be obtained abidirectional shift register which may perform stable bidirectionalshift operation with a simple circuit structure and an image displaydevice using the bidirectional shift register.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic diagram illustrating a structure of an imagedisplay device according to a first embodiment;

FIG. 2 is a schematic diagram illustrating a structure of abidirectional shift register according to the first embodiment;

FIG. 3 is a schematic circuit diagram of a unit register circuit in afirst stage of the bidirectional shift register according to the firstembodiment;

FIG. 4 is a schematic circuit diagram of a unit register circuit in asecond stage of the bidirectional shift register according to the firstembodiment;

FIG. 5 is a schematic circuit diagram of a unit register circuit in aλth stage of the bidirectional shift register according to the firstembodiment;

FIG. 6 is a schematic circuit diagram of a unit register circuit in an(N−1)th stage of the bidirectional shift register according to the firstembodiment;

FIG. 7 is a schematic circuit diagram of a unit register circuit in anN-th stage of the bidirectional shift register according to the firstembodiment;

FIG. 8 is a timing diagram illustrating waveforms of various signals inforward shift operation of the bidirectional shift register according tothe first embodiment;

FIG. 9 is a timing diagram illustrating waveforms of various signals inbackward shift operation of the bidirectional shift register accordingto the first embodiment;

FIG. 10 is a schematic diagram illustrating a structure of an imagedisplay device according to a second embodiment;

FIG. 11 is a schematic diagram illustrating a structure of abidirectional shift register according to the second embodiment;

FIG. 12 is a schematic diagram illustrating a structure of abidirectional shift register according to a third embodiment;

FIG. 13 is a schematic circuit diagram of a unit register circuit of thebidirectional shift register according to the third embodiment;

FIG. 14 is a circuit diagram illustrating a structure of a conventionalunit register circuit; and

FIG. 15 is a signal waveform chart illustrating operation of theconventional unit register circuit.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention (hereinafter simplyreferred to as embodiments) are described with reference to the attacheddrawings.

[First Embodiment]

FIG. 1 is a schematic diagram illustrating a structure of an imagedisplay device 10 according to a first embodiment. The image displaydevice 10 is, for example, a liquid crystal display. The image displaydevice 10 includes a plurality of pixel circuits 12, gate line drivingcircuits 14, a data line driving circuit 16, and a control circuit 18.

The pixel circuits 12 are arranged in a display portion in matrix so asto correspond to pixels.

A plurality of gate signal lines 20 arranged in a vertical direction(column direction) are connected to the gate line driving circuits 14.Each of the gate signal lines 20 is connected to a plurality of pixelcircuits 12 arranged in a horizontal direction (row direction). The gateline driving circuits 14 output a gate signal to the plurality of gatesignal lines 20 in sequence to render pixel circuits 12 connected to thegate signal lines 20 data writable.

A plurality of data lines 22 arranged in the horizontal direction areconnected to the data line driving circuit 16. Each of the data lines 22is connected to a plurality of pixel circuits 12 arranged in thevertical direction. The data line driving circuit 16 generates, fromimage data corresponding to one scanning line, signals corresponding toa plurality of pixels forming the scanning line and outputs the signalsto the plurality of data lines 22. The pixel signals which are output tothe respective data lines 22 are written to pixel circuits 12 which arerendered writable by a gate signal, and the respective pixel circuits 12control the amount of light emitted from the pixels according to thewritten pixel signals.

The control circuit 18 controls operation of the gate line drivingcircuits 14 and of the data line driving circuit 16.

The image display device 10 includes as the gate line driving circuits14 a gate line driving circuit 14L provided on a left side of thedisplay portion and a gate line driving circuit 14R provided on a rightside of the display portion. The gate line driving circuit 14R suppliesa gate signal to odd-numbered gate signal lines 20 while the gate linedriving circuit 14L supplies a gate signal to even-numbered gate signallines 20. The gate line driving circuits 14 and the control circuit 18form the bidirectional shift register and the order of supplying a gatesignal to the gate signal lines 20 may be switched between a forwarddirection from a top to a bottom of the display portion and a reversedirection from the bottom to the top of the display portion.

FIG. 2 is a schematic diagram illustrating a structure of abidirectional shift register 30 used for scanning the gate signal lines20 of the image display device 10. The bidirectional shift register 30includes a shift register portion 32, a clock signal generating portion34, and a trigger signal generating portion 36. The shift registerportion 32 is provided in the gate line driving circuits 14 while theclock signal generating portion 34 and the trigger signal generatingportion 36 are provided in, for example, the control circuit 18. Theshift register portion 32 includes cascaded unit register circuits 38 ina plurality of stages.

FIG. 2 illustrates by way of example a portion of the bidirectionalshift register 30 which relates to the shift register portion 32provided in the gate line driving circuit 14R on the right side. Thegate line driving circuit 14R drives in sequence the odd-numbered gatesignal lines 20, that is, every other gate signal lines 20 at timingsshifted by 2H (H is a horizontal scanning period for one row). On theother hand, the gate line driving circuit 14L drives in sequence theeven-numbered gate signal lines 20 at timings shifted by 1H from theodd-numbered gate signal lines 20. The shift register portion 32 of agate line driving circuit 14 on one side is adapted to be driven byfour-phase clock signals. The drive on one side is shifted by 1H fromthe other side, and thus, the clock signal generating portion 34generates eight-phase clock signals V1-V8. In each of the clock signals,a pulse having a cycle of 8H and a width of 2H is generated, and clocksignals which are adjacent to each other in terms of phase, that is, Vjand V(j+1) are set to have a phase difference of 1H. More specifically,clock pulses which are adjacent to each other in terms of phase overlapby 1H of a 2H period during which the clock pulses are held at an Hlevel. The clock signal generating portion 34 has a first set of signalsV1, V3, V5, and V7 and a second set of signals V2, V4, V6, and V8. Inboth of the sets, the signals are shifted by 2H. The clock signalgenerating portion 34 supplies the first set to the gate line drivingcircuit 14R and supplies the second set to the gate line driving circuit14L. The unit register circuit 38 in each of the stages is correlatedwith one clock signal (output control clock signal) having a phase thatdefines the timing of an output pulse in the stage among the clocksignals of the plurality of phases.

In forward shift operation of the shift register portion 32, the clocksignal generating portion 34 generates clock pulses in sequence in theforward direction, that is, in the order of V1, V2, . . . , V8, V1, . .. . On the other hand, in backward shift operation, the clock signalgenerating portion 34 generates clock pulses in sequence in the reversedirection, that is, in the order of V8, V7, . . . , V1, V8, . . . . Theclock signal generating portion 34 supplies the generated clock pulsesto the respective stages of the shift register portions 32 of the gateline driving circuit 14R and the gate line driving circuit 14L,respectively. The clock signal generating portion 34 supplies clocksignals in the order of V1, V3, V5, V7, V1, . . . with different phasesfor the respective stages as the output control clock signals to thegate line driving circuit 14R from the top stage (upper side) down tothe bottom stage (lower side). With regard to the gate line drivingcircuit 14L, the order is set to be V2, V4, V6, V8, V2, . . . .

The trigger signal generating portion 36 generates a forward directiontrigger signal VSTF at the start of a forward shift and generates areverse direction trigger signal VSTB at the start of a backward shift.More specifically, the trigger signal generating portion 36 generates apulse which rises to the H level in the signal VSTF at the start of aforward shift and generates a pulse which rises to the H level in thesignal VSTB at the start of a backward shift.

As described above, the shift register portion 32 is constructed toinclude the plurality of cascaded unit register circuits 38. Each of theunit register circuits 38 outputs a pulse from an output terminalthereof. The shift register portion 32 outputs a pulse from the unitregister circuits 38 in the respective stages. In the forward shiftoperation, the shift register portion 32 outputs a pulse in sequencefrom the top stage of the unit register circuits 38, while, in thebackward shift operation, the shift register portion 32 outputs a pulsein sequence from the bottom stage of the unit register circuits 38.

The unit register circuits 38 in the plurality of stages which form theshift register portion 32 include main stages having output terminalsthat are connected to the gate signal lines 20, respectively, and dummystages which are added to the top and the bottom of the column of themain stages and which are not connected to the gate signal lines 20. Thetotal number of stages in the shift register portion 32 is denoted as N.The value N of the total number of stages is determined by the number ofthe scanning lines of the image display device 10, that is, the numberof the gate signal lines 20, and the number of the top dummy stages andthe bottom dummy stages. In this embodiment, two dummy stages areprovided at the top and two dummy stages are provided at the bottom.When an output of the unit register circuit 38 in a k-th stage on thegate line driving circuit 14R side is expressed as G(2k−1) (k is anatural number which satisfies 1=k≦N), outputs G1, G3, G(2N−3), andG(2N−1) from the dummy stages are not output to the gate signal lines 20while an output G(2λ−1) from a λth stage (λ is a natural number whichsatisfies 3≦λ≦N−2) which is a main stage is output to a correspondinggate signal line 20.

It is to be noted that, when an output of the unit register circuit 38in the k-th stage on the gate line driving circuit 14L side is expressedas G(2k), outputs G2, G4, G(2N−2), and G(2N) from the dummy stages arenot output to the gate signal lines 20 while an output G(2λ) from theλth stage which is a main stage is output to a corresponding gate signalline 20.

FIG. 2 illustrates connections of input/output terminals of the unitregister circuits 38. It is to be noted that, for simple illustration,clock signals are denoted as, for example, V(2λ−1), where a clock signalVζ having a phase that is expressed by a number ζ exceeding 8 means aclock signal Vξ having a phase that is expressed by a remainder ξ leftwhen ζ is divided by 8.

FIGS. 3 to 7 are schematic circuit diagrams of the unit registercircuits 38. FIG. 3, FIG. 4, FIG. 5, FIG. 6, and FIG. 7 illustrate theunit register circuits 38 in a first stage, a second stage, the λthstage, an (N−1)th stage, and an N-th stage, respectively.

First, a basic structure of the unit register circuit 38 in the k-thstage (1≦k≦N) is described with reference to the unit register circuit38 in a main stage (λth stage) illustrated in FIG. 5, and then, astructure of the unit register circuit 38 in the dummy stages (k=1, 2,N−1, and N) is described mainly with reference to points different fromthose of the basic structure.

The unit register circuit 38 in the k-th stage includes n-channeltransistors T1F, T1B, T2 to T6, T7F, T7B, T9F, T9B, T10F, and T10B andcapacitors C1 and C3.

The unit register circuit 38 in the k-th stage has an output terminalNOUT(k) for outputting a pulse G(2k−1) of its own stage. The unitregister circuit 38 in the k-th stage further has, as terminals to whicha pulse or a trigger signal in another stage is input, a forwarddirection set terminal NSF(k), a reverse direction set terminal NSB(k),a forward direction reset terminal NRF(k), and a reverse direction resetterminal NRB(k). An output signal G(2λ−3) is input to a terminal NSF(λ)in a main stage from a (λ−1)th stage, an output signal G(2λ+1) is inputto a terminal NSB(λ) from a (λ+1)th stage, an output signal G(2λ+3) isinput to a terminal NRF(λ) from a (λ+2)th stage, and an output signalG(2λ−5) is input to a terminal NRB(λ) from a (λ−2)th stage. Some of theinput terminals of the dummy stages do not have corresponding outputsignals in other stages. If so, a trigger signal is input to such aterminal. The dummy stages are to be described more specifically below.

Further, V(2k−1) and V(2k+3) are input to the unit register circuit 38in the k-th stage from the clock signal generating portion 34.

Further, to the unit register circuits 38, the forward direction triggersignal VSTF and the reverse direction trigger signal VSTB are input fromthe trigger signal generating portion 36, and a voltage at the H levelis supplied from a power supply VGH and a voltage at the L level issupplied from a power supply VGL.

A drain of the output transistor T5 is connected to a signal line of theoutput control clock signal V(2k−1) and a source of the outputtransistor T5 is connected to the output terminal NOUT(k), and theconduction of the transistor T5 is controlled according to the potentialof a reference point N1 connected to a gate of the transistor T5. Thecapacitor C1 is connected between the gate and the source of thetransistor T5. The transistor T5 and the capacitor C1 function as anoutput circuit which outputs the output pulse G(2k−1) of its own stagein synchronization with the clock pulse V(2k−1) with the node N1 as thereference point being at the H level.

Further, a drain of the transistor T6 is connected to the outputterminal NOUT(k) and a source of the transistor T6 is connected to thepower supply VGL, and on/off of the transistor T6 is controlledaccording to the potential of a node N2 connected to a gate of thetransistor T6. The capacitor C3 is connected between the node N2 and thepower supply VGL.

The reference point N1 is connected to the terminals NSF(k) and NSB (k)via the diode-connected transistors T1F and T1B, respectively. Thetransistors T1F and T1B function as a set circuit which sets thereference point N1 to the H level when an output pulse of another stageis input to the terminal NSF(k) or NSB(k).

The transistors T2, T9F, and T9B which are connected between thereference point N1 and the power supply VGL so as to be in parallel toone another function as switch elements which providecontinuity/discontinuity between the reference point N1 and the powersupply VGL. A gate of the transistor T2 is connected to the node N2, agate of the transistor T9F is connected to the terminal NRF(k), and agate of the transistor T9B is connected to the terminal NRB(k). When thepotential of the node N2, the terminal NRF(k), or the terminal NRB(k) isat the H level, these transistors set the potential of the referencepoint N1 to the L level. In particular, the transistors T9F and T9Bfunction as a reset circuit which sets the reference point N1 to the Llevel when an output pulse of another stage is input to the terminalNRF(k) or NRB(k).

Here, the node N2 is set to the H level except for a period during whichthe reference point N1 is set to the H level. The transistor T2 is ONduring a period in which the node N2 is at the H level, and thus, thecontinuity is maintained for a relatively long time. As a result, athreshold voltage Vth of the transistor T2 is shifted to the positivedirection, and the ability of the transistor T2 to fix the referencepoint N1 at the L level is lowered. On the other hand, a pulse of theclock signal V(2k−1) is applied to the drain of the transistor T5 evenoutside the set period of the reference point N1 (output period of thek-th stage), and the pulse has a function to pull up the potential ofthe reference point N1 via a gate-drain capacitance Cgd of thetransistor T5. In particular, as described below, at least the size ofthe transistors T5 in the main stages is required to be large, and thus,the gate-drain capacitance Cgd becomes larger accordingly and thepulled-up amount of the potential of the reference point N1 becomeslarger accordingly. Therefore, the transistors T9F and T9B are providedso that the reference point N1 is preferably reset to the L level.

The transistors T3, T10F, and T10B which are connected between the nodeN2 and the power supply VGH so as to be in parallel to one anotherfunction as switch elements which provide continuity/discontinuitybetween the node N2 and the power supply VGH. A gate of the transistorT3 is connected to a signal line of a clock signal (2k+3), a gate of thetransistor T10F is connected to a signal line of the forward directiontrigger signal VSTF, and a gate of the transistor T10B is connected to asignal line of the reverse direction trigger signal VSTB. When thepotential of the clock signal (2k+3), the forward direction triggersignal VSTF, or the reverse direction trigger signal VSTB is at the Hlevel, these transistors set the potential of the node N2 at the Hlevel.

The transistors T4, T7F, and T7B which are connected between the node N2and the power supply VGL so as to be in parallel to one another functionas switch elements which provide continuity/discontinuity between thenode N2 and the power supply VGL. A gate of the transistor T4 isconnected to the node N1, a gate of the transistor T7F is connected tothe terminal NSF(k), and a gate of the transistor T7B is connected tothe terminal NSB(k). When the potential of the node N1, the terminalNSF(k), or the terminal NSB(k) is at the H level, these transistors setthe potential of the node N2 at the L level.

Next, the unit register circuits 38 in the dummy stages are described.The unit register circuit 38 in the first stage illustrated in FIG. 3 isdifferent from the unit register circuit 38 illustrated in FIG. 5 in nothaving the transistor T10F. The unit register circuit 38 in the N-thstage illustrated in FIG. 7 is different from the unit register circuit38 illustrated in FIG. 5 in not having the transistor T10B. The unitregister circuit 38 in the second stage illustrated in FIG. 4 and theunit register circuit 38 in the (N−1)th stage illustrated in FIG. 6 aresimilar to the unit register circuit 38 illustrated in FIG. 5 withregard to the basic structure of the circuit.

As described above, with regard to the dummy stages, there are cases inwhich no output signal in another stage for supplying an output pulse tothe terminal NSF, NSB, NRF, or NRB exists. More specifically, terminalshaving no output signal from another stage are NSF in the first stage,NRB in the first and second stages, NSB in the N-th stage, and NRF inthe (N−1)th and N-th stages.

Among these, the set terminals NSF and NSB are used for inputting asignal which sets the reference point N1 to the H level in preparationfor generating an output pulse. Therefore, to the set terminals NSF inthe first stage, a pulse of the forward direction trigger signal VSTF isinput from the trigger signal generating portion 36 at the start of aforward shift. Further, to the set terminals NSB in the N-th stage, apulse of the reverse direction trigger signal VSTB is input at the startof a backward shift.

On the other hand, the reset terminals NRF and NRB are used forinputting a signal which resets the reference point N1 to the L levelafter an output pulse is generated. By resetting the reference point N1to the L level, an output pulse is prevented from being generated by apulse of an output control clock signal which is input after that. Here,outputs of the dummy stages are not used for driving the gate signallines 20. Further, outputs of the (N−1)th and N-th stages which aredummy stages that operate after output pulses in the main stages aregenerated in a forward shift, and outputs of the first and second stageswhich are dummy stages that operate after output pulses in the mainstages are generated in a backward shift are not used as signals forsetting the reference point N1 in another stage. Therefore, these dummystages which operate at the end of each shift operation do not presentany specific problem even if the dummy stages repeatedly generate outputpulses according to repetition of clock pulses. Therefore, it issufficient that a signal of some kind at the H level is input to theterminals NRF in the (N−1)th and N-th stages in a forward shift and tothe terminals NRB in the first and second stages in a backward shiftbefore the start of shift operation with regard to the next frame tomake the reference point N1 in the stage concerned enter a reset state.As an example, in this embodiment, the forward direction trigger signalVSTF is input to the terminals NRF in the (N−1)th and N-th stages whilethe reverse direction trigger signal VSTB is input to the terminals NRBin the first and second stages.

The output terminal NOUT in a main stage is connected to the gate signalline 20 and to the plurality of pixel circuits 12 as loads to be driven.As the length of the gate signal line 20 increases due to a largerscreen and as the number of the pixel circuits 12 connected to the gatesignal line 20 increases due to higher resolution, the loads to bedriven become heavier. The output transistor T5 in a main stage isrequired to have a driving ability corresponding to the loads, and isdesigned to have, for example, a large gate width (channel width). Forexample, the output transistor T5 in a main stage is designed to have achannel width as large as about 5,000 μm. On the other hand, dummystages are not connected to the gate signal lines 20, and thus, thedriving ability of the output transistor T5 in a dummy stage is set tobe lower than that in a main stage. For example, the output transistorT5 in a dummy stage is set to have a channel width of about 500 μm whichis 1/10 of that of the output transistor T5 in a main stage. In thisway, the size of the transistors T5 in the dummy stages reduces, andthus, the unit register circuits 38 in the dummy stages may beminiaturized. Further, power consumption of the dummy stages may bereduced.

In the above, the structure of the gate line driving circuits 14 isdescribed taking as an example the gate line driving circuit 14R on theright side for driving the odd-numbered gate signal lines 20. Astructure of the gate line driving circuit 14L on the left side fordriving the even-numbered gate signal lines 20 is similar to that of thegate line driving circuit 14R on the right side, but is describedbriefly for confirmation. For example, the λth stage of the shiftregister portion 32 is connected to the gate signal line 20 in the(2λ−1)th row in the gate line driving circuit 14R on the right side,while the λth stage of the shift register portion 32 is connected to thegate signal line 20 in the (2λ)th row in the gate line driving circuit14L on the left side. In a forward shift, the gate signal line 20 in the(2λ)th row is driven with the delay of 1H from the drive of the gatesignal line 20 in the (2λ−1)th row. As is easily conceived from thisrelationship, the output terminal NOUT(k) of the unit register circuit38 in the k-th stage of the gate line driving circuit 14L outputs asignal G(2k), and G(2λ−2), G(2λ+2), G(2λ+4), and G(2λ−4) are input tothe terminals NSF(λ), NSB(λ), NRF(λ), and NRB(λ), respectively, in themain stages. Further, V(2k) as the output control clock signal is inputto the transistor T5, and a clock signal V(2k+4) is applied to the gateof the transistor T3.

Next, operation of the bidirectional shift register 30 is described.FIG. 8 is a timing diagram illustrating waveforms of various signals inthe forward shift operation.

The forward shift starts by, at the head of image signals for one frame,generation of pulses of forward direction trigger signals by the triggersignal generating portion 36 (at times t0 and t1). The trigger signalgenerating portion 36 generates, after generating a pulse of the forwarddirection trigger signal VSTF for driving the odd-numbered lines at thetime t0, a pulse of a forward direction trigger signal VSTF2 for drivingthe even-numbered lines at the time t1, which is delayed by 1H from t0.On the other hand, the reverse direction trigger signal VSTB for drivingthe odd-numbered lines and a reverse direction trigger signal VSTB2 fordriving the even-numbered lines are fixed at the L level.

In the unit register circuits 38 in the second to N-th stages, when apulse of the signal VSTF is input, the transistor T10F is turned on, thenode N2 is pulled up to the H level, and, as a result, the transistor T2is turned on to reset the reference point N1 to the L level. On theother hand, in the unit register circuits 38 in the first to (N−αb)thstages, when a pulse of the signal VSTB is input, the transistor T10B isturned on to reset the reference point N1 to the L level.

As described above, the clock signal generating portion 34 generates, inthe forward shift operation, the pulses in sequence in the forwarddirection. More specifically, the clock signal generating portion 34raises a pulse of a clock signal V(j+1) with a delay of 1H from a risingedge of a pulse of a clock signal Vj (j is a natural number whichsatisfies 1≦j≦7), and further, raises a pulse of the clock signal V1with a delay of 1H from a rising edge of a pulse of the clock signal V8.

Here, first, forward shift operation of the unit register circuit 38 ina main stage (λth stage) in the gate line driving circuit 14R isdescribed.

Before the operation in the λth stage, the first to (λ−1)th stages areoperated in sequence to output pulses having a width of 2H with a phasedifference of 2H therebetween. When a pulse of the output signal G(2λ−3)in the (λ−1)th stage is input to the terminal NSF(λ) (time t2), thereference point N1 is set to a potential (VGH-Vth (T1F)) correspondingto the H level, the transistor T5 is turned on, and the interterminalvoltage of the capacitor C1 is set to the same potential. Here, thetransistor T4 is turned on to set the node N2 to the L level. Further,here, the transistor T7F is also turned on, and thus, the node N2 is setto the L level faster than in a case in which only the transistor T4 isturned on. The potential of the node N2 is held in the capacitor C3. Thenode N2 is at the L level, and thus, the transistors T2 and T6 are in anoff state.

An output pulse in the (λ−1)th stage is generated in synchronizationwith a pulse of a clock V(2λ−3) (in FIG. 8, a pulse of the clock V7),and thus, at a time t3 which is delayed by 2H from the time t2, a pulseof a clock signal V(2λ−1) (in FIG. 8, a pulse of the clock signal V1) isinput to the λth stage. The pulse of the clock signal V(2λ−1) pulls upthe potential of the source of the transistor T5. Then, the potential ofthe node N1 is further pulled up by the bootstrap effect, and the pulseof the clock signal V(2λ−1) becomes a pulse of the signal G(2λ−1)without lowering the potential thereof to be output from the terminalNOUT(λ). The pulse of the signal G(2λ−1) is input to the terminal NSF inthe (λ+1)th stage, and sets the node N1 in that stage to the H level.

When, at a time t4, the pulse of the clock signal V(2λ−1) falls, thepulse of the signal G(2λ−1) also falls. On the other hand, the potentialof the reference point N1 is held at the H level.

At the time t4, the (λ+1)th stage outputs a pulse of the signal G(2λ+1)in synchronization with a pulse of a clock signal V(2λ+1). In this way,each of the stages outputs a pulse of its own with a delay of 2H fromthe output of a pulse in the preceding stage. The (λ+2)th stage whichreceives the output of the pulse in the (λ+1)th stage outputs a pulse ofthe signal G(2λ+3) at a time t5 which is delayed by 2H from the time t4.

In the λth stage, when, at the time t5, the pulse of the signal G(2λ+3)is input to the terminal NRF, the transistor T9F is turned on to resetthe reference point N1 to the L level. At the same time, a clock signalV(2λ+3) turns on the transistor T3 to pull up the node N2 to the Hlevel. As a result, the transistor T6 is turned on to connect the outputterminal NOUT(λ) to the power supply VGL.

It is to be noted that the transistor T3 is periodically turned on bythe clock signal V(2λ+3) also at times other than the time t5, andsatisfactorily holds the node N2 at the H level except for a period inwhich the reference point N1 is in a set state. This causes the outputterminal NOUT(λ) to be held at the L level except for the period inwhich the reference point N1 is set to the H level.

By the above-mentioned operation, during the period of 2H which precedesthe time t2, a pulse is input from the (λ−2)th stage to the terminalNRB(λ) to turn on the transistor T9B. The period is before the referencepoint N1 is set to the H level by the input of a pulse from the (λ−1)thstage to the terminal NSF(λ), and thus, the above-mentioned operation isnot affected. Further, during the period of 2H between the times t4 andt5, a pulse is input from the (λ+1)th stage to the terminal NSB(λ), anda potential at the H level is applied from the terminal NSB(λ) via thetransistor T1B to the reference point N1. The period is before thereference point N1 is reset to the L level by the input of a pulse fromthe (λ+2)th stage to the terminal NRF(λ), and thus, the above-mentionedoperation is not affected.

Further, the timing of setting the reference point N1 to the H level isafter a pulse which precedes a pulse at the time t3 by a cycle among theplurality of pulses of the clock signal V(2λ−1), and the timing ofresetting the reference point N1 to the L level is before a pulse whichis generated after a cycle, and therefore, a pulse is output from theterminal NOUT(λ) only once, which is in synchronization with the pulseat the time t3 among the pulses of the clock signal V(2λ−1).

As described above, a main stage receives an output pulse from theprevious stage to cause the reference point N1 to be in the set state,and receives an output pulse from the stage next to the subsequent stageto cause the reference point N1 to be in the reset state. However, thedummy stage as the first stage does not have the previous stage.Therefore, as described above, the first stage has a structure in whicha pulse of the forward direction trigger signal VSTF is input to theterminal NSF. The first stage receives a pulse of the signal VSTF whichis generated at the time t0 to set the reference point N1 to the Hlevel. Subsequent operation in the first stage is similar to that in theλth stage described above. Further, the dummy stages as the (N−1)th andN-th stages do not have the stage next to the subsequent stage.Therefore, as described above, the (N−1)th and N-th stages have astructure in which a pulse of the forward direction trigger signal VSTFis input to the terminal NRF. The reference point N1 in the (N−1)th andN-th stages is set to the H level at the end of the forward shiftoperation for one frame, and then, in response to a pulse of the signalVSTF which is generated at the start of the subsequent frame, is resetto the L level.

The forward shift operation of the stages in the gate line drivingcircuit 14R is described above. The forward shift operation of thestages in the gate line driving circuit 14L is similar to that of thecorresponding stages in the gate line driving circuit 14R. However, itis to be noted that the stages in the gate line driving circuit 14Loperate with a delay of 1H from the corresponding stages in the gateline driving circuit 14R, respectively.

FIG. 9 is a timing diagram illustrating waveforms of various signals inthe backward shift operation.

The backward shift starts by, at the head of image signals for oneframe, generation of pulses of reverse direction trigger signals by thetrigger signal generating portion 36 (at times t0 and t1). The triggersignal generating portion 36 generates, after generating a pulse of thereverse direction trigger signal VSTB2 for driving the even-numberedlines at the time t0, the pulse of a reverse direction trigger signalVSTB for driving the odd-numbered lines at the time t1, which is delayedby 1H from t0. On the other hand, the forward direction trigger signalVSTF for driving the odd-numbered lines and the forward directiontrigger signal VSTF2 for driving the even-numbered lines are fixed atthe L level.

As described above, the clock signal generating portion 34 generates, inthe backward shift operation, the pulses in sequence in the reversedirection. More specifically, the clock signal generating portion 34raises the pulse of the clock signal Vj with a delay of 1H from a risingedge of the pulse of the clock signal V(j+1), and further, raises apulse of the clock signal V8 with a delay of 1H from a rising edge ofthe pulse of the clock signal V1.

The unit register circuit 38 in each of the stages of the shift registerportion 32 has a circuit structure in which a portion related to theterminal NSF and a portion related to the terminal NSB are symmetricalwith each other and in which a portion related to the terminal NRF and aportion related to the terminal NRB are symmetrical with each other.More specifically, according to the four-phase clocks used for drivingthe gate line driving circuit 14 on one side, in both the forward shiftoperation and the backward shift operation, the unit register circuits38 in each of the stages is adapted to receive at the terminal NSB anoutput pulse which is generated with an advance of one phase of theclock, that is, 2H, from that of its own stage and causes the referencepoint N1 to be in the set state, and is adapted to receive at theterminal NRB an output pulse which is generated with a delay of twophases of the clock, that is, 4H, from that of its own stage and causesthe reference point N1 to be in the reset state. Further, both ends ofthe shift register portion 32, that is, the top dummy stages and thebottom dummy stages are in a symmetrical relationship in structure withrespect to shifts in opposite directions. More specifically, the topdummy stages in the backward shift operation function similarly to thebottom dummy stages in the forward shift operation, while the bottomdummy stages in the backward shift operation function similarly to thetop dummy stages in the forward shift operation. Therefore, if thecontrol circuit 18 switches the trigger signals and switches the orderof generation of the clock pulses, the shift register portion 32performs the backward shift operation similarly to the forward shiftoperation.

For example, in the N-th stage in the gate line driving circuit 14R, apulse of the reverse direction trigger signal VSTB is input to theterminal NSB at the time t1 and the reference point N1 is set to the Hlevel. After that, in synchronization with a pulse of a clock signalV(2N−1) which is generated first, a pulse is generated in an outputsignal G(2N−1). In this way, pulses are output from the stages insequence in the opposite order to that in the forward shift operation.

The backward shift operation is described in the above taking as anexample the gate line driving circuit 14R. The backward shift operationof each of the stages in the gate line driving circuit 14L is similar tothat of the corresponding stage in the gate line driving circuit 14R.However, each of the stages in the gate line driving circuit 14Lperforms operation with an advance of 1H from the corresponding stage inthe gate line driving circuit 14R.

Here, with its own stage being a starting point, another stage whichinputs a pulse to the reset terminal NRF is set to be a stage which isfarther than still another stage that inputs a pulse to the set terminalNSB, and, another stage which inputs a pulse to the reset terminal NRBis set to be a stage which is farther than still another stage thatinputs a pulse to the set terminal NSF. In this structure, in theforward shift operation, the pulses which are input to the terminals NSBand NRB that are related to the backward shift operation do not affectthe forward shift operation, and, similarly, in the backward shiftoperation, the pulses which are input to the terminals NSF and NRF thatare related to the forward shift operation do not affect the backwardshift operation. Therefore, for example, it is not necessary to providea switch for selectively accepting the inputs to the terminals NSF andNRF in the forward shift operation and selectively accepting the inputsto the terminals NSB and NRB in the backward shift operation. Morespecifically, the shift register portion 32 and the unit registercircuits 38 therein may have a basic circuit structure which is notswitched between one for the forward shift and one for the backwardshift. A transistor used as the switch is not necessary, and thus, thecircuit structure of the unit register circuit 38 becomes simpleraccordingly, which makes it easier to miniaturize the unit registercircuit 38. Further, a signal line for supplying a switching signal tothe transistor concerned in each of the stages is not required to bearranged along the shift register portion 32, and thus, the sizeincrease in a horizontal direction of the gate line driving circuits 14may be suppressed.

It is to be noted that, as described in the description of the forwardshift operation, in synchronization with the operation of resetting thereference point N1, a clock signal is used to turn on the transistor T3and the node N2 is pulled up to the H level. In this embodiment, clocksfor driving a gate line driving circuit 14 on one side are four-phaseclocks, and, for example, in the gate line driving circuit 14R, thereference point N1 is reset to a timing which is delayed by two phasesof the clock from the output control clock signal V(2k−1) to the outputtransistor T5 in its own stage. The clock signal which turns on thetransistor T3 at the timing of resetting the reference point N1 isV(2k+3) in the forward shift and V(2k−5) in the backward shift, whichare a common clock signal. This means that, in this embodiment, a clocksignal for controlling the transistor T3 is also not required to beswitched between one for the forward shift and one for the backwardshift.

By the way, in the embodiment described above, a gate line drivingcircuit 14 on one side is driven by four-phase clock signals, and,basically, outputs of the (k−2)th stage, the (k−1)th stage, the (k+1)thstage, and the (k+2)th stage are input to the unit register circuit 38in the k-th stage so that the reference point N1 is set to the H levelby output pulses in (k−1)th stage and the (k+1)th stage and that thereference point N1 is reset to the L level by output pulses in the(k−2)th stage and the (k+2)th stage. Such a structure materializes abidirectional shift register which does not basically require switchbetween a circuit structure for the forward shift and a circuitstructure for the backward shift. Further, in such a structure, after anoutput pulse of each of the stages falls, the reference point N1 isreset from the H level to the L level. More specifically, after anoutput pulse of each of the stages ends, a subsequent set period inwhich the reference point N1 in the stage is held in the set state isprovided. By the existence of the subsequent set period, operation ofthe bidirectional shift register according to the present invention isnot operation in which the potential of the reference point N1 isabruptly pulled down from a potential higher than the H level to the Llevel and the transistor M6 is turned on, and thus, unstable operationdue to timing misalignment and deformation of the waveform of each ofthe signals such as a through current is less liable to occur. Here, thepresent invention is not limited to the structure of the above-mentionedembodiment. A generalized structure of the bidirectional shift registeraccording to the present invention is as follows. It is assumed that theshift register portion 32 is driven by n-phase clock signals (n is aninteger which is equal to or larger than 3), and αf, αb, βf, and βb arenatural numbers which satisfy both αf<βb<n and αb<βf<n. Then, outputs ofa (k−βb)th stage, a (k−αf)th stage, a (k+αb)th stage, and a (k+βf)thstage are input to the unit register circuit 38 in the k-th stage sothat the reference point N1 is set to the H level by output pulses inthe (k−αf)th stage and the (k+αb)th stage and that the reference pointN1 is reset to the L level by output pulses in the (k−βb)th stage andthe (k+βf)th stage. Such a structure also materializes a bidirectionalshift register which does not basically require switch between circuitstructures with the improved stability of operation.

It is to be noted that, because αf<βb and αb<βf, it follows that both βfand βb are equal to or larger than 2. Then, it follows that the value Nof the total number of stages is equal to or larger than 3. However, ina bidirectional shift register having βb dummy stages at the top and βfdummy stages at the bottom as in the above-mentioned embodiment, atleast two main stages are necessary for the forward shift and thebackward shift, and thus, it follows that N is equal to or larger than6.

It is to be noted that, in some cases, similarly to the case of theabove-mentioned dummy stages, a signal as an alternative an output pulsein another stage is input to the terminal NSF, NSB, NRF, or NRB of theunit register circuits 38 at both ends of the generalized shift registerportion 32. More specifically, with regard to a bidirectional shiftregister having N stages, in the unit register circuits 38 in the firstto αf-th stages, the forward direction trigger signal is input to theterminal NSF, and the signal sets the reference point N1 to the H levelat the start of the forward shift. In the unit register circuits 38 inthe (N−αf+1)th to N-th stages, the reverse direction trigger signal isinput to the terminal NSB, and the signal causes the reference point N1to be in the set state at the start of the backward shift. Further, as areset signal to be input to the terminal NRF in the (N−βf+1)th to N-thstages, the forward direction trigger signal may be used. As a resetsignal to be input to the terminal NRB in the first to βb-th stages, thereverse direction trigger signal may be used.

The basic structure of the unit register circuit 38 may include thetransistors T10F and T2 as a circuit for setting the reference point N1to the L level as an initial setting by a pulse of the forward directiontrigger signal VSTF. However, as described above, in the first to αf-thstages, a pulse of the signal VSTF is used for setting the referencepoint N1 to the H level. Therefore, in the unit register circuits 38 inthe first to αf-th stages, the circuit structure does not have thetransistor T10F similarly to the case of the first stage in theabove-mentioned embodiment (see FIG. 3) so that the reference point N1is not reset to the L level. Similarly, the basic structure of the unitregister circuit 38 may include the transistors T10B and T2 as a circuitfor setting the reference point N1 to the L level as the initial settingby a pulse of the reverse direction trigger signal VSTB. However, asdescribed above, in the (N−αb+1)th to N-th stages, a pulse of the signalVSTB is used for setting N1 to the H level. Therefore, in the unitregister circuits 38 in the (N−αb+1)th to N-th stages, the circuitstructure does not have the transistor T10B similarly to the case of theN-th stage in the above-mentioned embodiment (see FIG. 7) so that thereference point N1 is not reset to the L level.

αf corresponds to a period from when the reference point N1 is set towhen an output pulse rises (preceding set period) in the forward shiftoperation while αb corresponds to the preceding set period in thebackward shift operation. When the preceding set period becomes long,the potential of the reference point N1 held by the capacitor C1 may belowered by a leakage current of the transistor T9F or T9B or the like,which may result in an inconvenience that, when a clock pulse is inputto the drain of the transistor T5, the potential of the gate of thetransistor T5 is not high enough to output a pulse from the terminalNOUT. Therefore, when there is concern about the above-mentionedinconvenience, for example, when the capacitance of the capacitor C1 isnot so large, it is preferred that, as in the above-mentionedembodiment, αf and αb be set to 1 to make short the preceding setperiod.

Further, from the viewpoint of symmetry between operation of the imagedisplay device 10 in the forward shift operation and that in thebackward shift operation, it is preferred that αf=αb and βf=βb besatisfied.

In the above-mentioned embodiment where n=4 and βf=βb=2, as describedabove, a clock signal used for controlling the transistor T3 may becommon between the forward shift operation and the backward shiftoperation. Such a structure in which a clock signal for controlling thetransistor T3 is common to the two directions is materialized whenβf+βb=n.

In the above-mentioned embodiment, the reverse direction trigger signalVSTB is fixed to the L level in the forward shift operation, and theforward direction trigger signal VSTF is fixed to the L level in thebackward shift operation. In this driving method, in the forward shiftoperation, the transistor T9B having a gate connected to the reversedirection reset terminal NRB in the first to βb-th stages is held in theoff state, and, in the backward shift operation, the transistor 19Fhaving a gate connected to the forward direction reset terminal NRF inthe (N−βf+1)th to N-th stages is held in the off state. Such atransistor in which a voltage is applied between the drain and thesource thereof and which is held in the off state for a long time mayhave a change in transistor characteristics which is called Vth shift.More specifically, in an n-channel transistor, the threshold voltage Vthis liable to be lowered and a leakage current is liable to occur. Such aVth shift particularly presents a problem in an a-Si thin filmtransistor (TFT). For example, it is known that a Vth shift in atransistor may be eliminated by once turning on the transistor to causea current to flow therethrough.

Therefore, in the method of driving the shift register portion 32 in theabove-mentioned embodiment, when the forward shift is repeated over aplurality of frames, the trigger signal generating portion 36 may switchthe signal VSTB to the H level during a period between the repeatedoperation to turn on the transistor T9B. On the other hand, when thebackward shift is repeated, the trigger signal generating portion 36 mayswitch the signal VSTF to the H level during a period between therepeated operation to turn on the transistor T9F. This may prevent thepotential of the reference point N1 from being lowered during a periodof being set to the H level due to a leakage current of the transistorT9F or T9B resulting from a Vth shift. It is to be noted that the periodbetween the repeated operation may be set, in the forward shiftoperation, in a range after an output pulse is generated in the N-thstage and before a pulse of the forward direction trigger signal VSTFwith regard to the next frame is generated, and, in the backward shiftoperation, in a range after an output pulse is generated in the firststage and before a pulse of the reverse direction trigger signal VSTBwith regard to the next frame is generated. For example, the period maybe set within a vertical blanking period of image signals written in thepixel circuits 12.

Further, the unit register circuits are not limited to the onesillustrated in FIGS. 3 to 7, and may have other circuit structures whichinclude the forward direction set terminal NSF and the reverse directionset terminal NSB, the forward direction reset terminal NRF and thereverse direction reset terminal NRB, a set circuit for setting thepotential of a reference point to a first potential when a set signal isinput to the terminal NSF or the terminal NSB, a reset circuit forsetting the potential of the reference point to a second potential whena reset signal is input to the terminal NRF or the terminal NRB, and anoutput circuit for outputting a pulse to an output signal insynchronization with a clock pulse which is input to the unit registercircuit with the reference point being at the first potential. Forexample, a unit register circuit 60 in a third embodiment describedbelow is an example of such a circuit structure. Further, when theabove-mentioned condition for controlling the transistor T3 by a clocksignal common between the forward shift and the backward shift (βf+βb=n)is not satisfied, a circuit structure may be adopted in which a controlsignal to be applied to the gate of the transistor T3 is switchedbetween one for the forward shift and one for the backward shift, andthis is one modification of the unit register circuit.

It is to be noted that, in the above-mentioned embodiment, a case isdescribed in which a pulse of a clock signal which drives a gate linedriving circuit 14 on one side does not overlap another pulse in anadjacent phase. However, the present invention may also be applied to acase in which a pulse of a clock signal overlaps another pulse in anadjacent phase. In that case, it is necessary that a set signal to theterminal NSF or NSB and a reset signal to the terminal NRF or NRB do notoverlap when being input. More specifically, it is necessary that anoutput pulse in the (k−βb)th stage and an output pulse in the (k−αf)thstage do not overlap each other and an output pulse in the (k+αb)thstage and an output pulse in the (k+βf)th stage do not overlap eachother. By this, the following condition of αf, αb, βf, and βb isobtained:αf+κ≦βb<n and αb+κ≦βf<n,where κ is the width of a clock pulse (κ≧1) when, for example, the phasedifference between the clock signals Vj and V(j+1) is 1.

A case in which an n-channel transistor is used as a transistor thatforms the bidirectional shift register 30 according to the presentinvention is described above, but the transistor may be a p-channel one.Further, the transistor may be TFT or may be MOSFET. A semiconductorlayer which forms the transistor basically may be any one ofmonocrystalline silicon, amorphous silicon (a-Si), and polycrystallinesilicon (poly-Si), and may also be an oxide semiconductor such as indiumgallium zinc oxide (IGZO).

[Second Embodiment]

In the following, like reference numerals are used to denote memberssimilar to those in the above-mentioned first embodiment and descriptionthereof is omitted for the sake of simplicity of description.

FIG. 10 is a schematic diagram illustrating a structure of an imagedisplay device 50 according to a second embodiment. Similarly to theimage display device 10, the image display device 50 is, for example, aliquid crystal display. The image display device 50 includes a pluralityof pixel circuits 12, gate line driving circuits 14, a data line drivingcircuit 16, and a control circuit 18.

The gate line driving circuits 14 on the right and on the left have thesame circuit structure and simultaneously operate in synchronizationwith each other under the control of the control circuit 18 to supply agate signal to all the gate signal lines 20 no matter which of anodd-numbered line and an even-numbered line the gate signal line 20 is.In other words, the gate line driving circuits 14 on the right and onthe left apply the same drive signal from both sides of the gate signalline 20. By applying a pulse from both sides in this way, effects ofdeterioration of the waveform of the drive signal due to CR of the gatesignal line 20 may be reduced. The gate line driving circuits 14 drivethe gate signal lines 20 at timings shifted by 1H.

FIG. 11 is a schematic diagram illustrating a structure of abidirectional shift register 30 used for scanning the gate signal lines20 of the image display device 50. N unit register circuits 38 havingthe same structure as in the first embodiment are cascaded in the shiftregister portion 32. The number of the main stages is N−4 and two dummystages are provided at the top before the main stages and two dummystages are provided at the bottom after the main stages. Thebidirectional shift register 30 has a circuit structure which isbasically the same as that described in the first embodiment withreference to FIG. 2. The difference is that output terminals in thestages of the shift register portion 32 are connected not to every othergate signal lines 20 but to all the gate signal lines 20.Correspondingly to this, while, in the first embodiment, an outputsignal in the k-th stage (k is a natural number which satisfies 1≦k≦N)of the gate line driving circuit 14R is denoted as G(2k−1) and an outputsignal in the k-th stage of the gate line driving circuit 14L is denotedas G(2k), an output signal in the k-th stage of the gate line drivingcircuits 14 of the image display device 50 is denoted as Gk. G1, G2,G(N−1), and GN which are outputs of the dummy stages are not output tothe gate signal lines 20, and an output Gλ in the λth stage (λ is anatural number which satisfies 3≦λ≦N−2) which is a main stage is outputto a gate signal line 20.

Operation of the bidirectional shift register 30 in this embodiment isbasically similar to that of the gate line driving circuit 14R of thefirst embodiment described with reference to FIGS. 8 and 9. However,while, in the first embodiment, the width of a clock pulse is 2H, thewidth of a clock pulse is 1H in this embodiment. With this, the gateline driving circuits 14 switch terminals which output a pulse insequence in the forward direction or in the reverse direction every 1Hperiod to drive the gate signal lines 20 in sequence.

It is to be noted that various kinds of variations in structuredescribed in the first embodiment may also be adopted in thebidirectional shift register according to this embodiment.

[Third Embodiment]

In the following, like reference numerals are used to denote memberssimilar to those in the above-mentioned first embodiment and descriptionthereof is omitted for the sake of simplicity of description.

A schematic structure of an image display device 10 according to a thirdembodiment is similar to that of the first embodiment described withreference to FIG. 1. The gate line driving circuit 14R drivesodd-numbered lines while the gate line driving circuit 14L driveseven-numbered lines.

FIG. 12 illustrates by way of example a structure of a portion whichrelates to the shift register portion 32 provided in the gate linedriving circuit 14R on the right side. N unit register circuits 60 arecascaded in the shift register portion 32. The number of the main stagesis N−4 and two dummy stages are provided at the top before the mainstages and two dummy stages are provided at the bottom after the mainstages. As described below, the unit register circuit 60 in thisembodiment has a circuit structure which is different from that of theunit register circuit 38 in the first embodiment in that a clock signaland a control signal to be input to each of the stages are differentfrom those in the first embodiment illustrated in FIG. 2. However,outputs of the stages and the way of cascading the stages are basicallysimilar to those in the first embodiment.

The clock signal generating portion 34 is similar to that in the firstembodiment. The eight-phase clock signals V1 to V8 are divided into twosets of four-phase clock signals. Among them, the group of V1, V3, V5,and V7 is supplied to the gate line driving circuit 14R. All of V1, V3,V5, and V7 are input to each of the unit register circuits 60. One clocksignal among them which is used as the output control clock signal isdetermined according to the location of the unit register circuit 60 inthe shift register portion 32.

FIG. 13 is a schematic circuit diagram of the unit register circuit 60.FIG. 13 illustrates the unit register circuit 60 in the λth stage (mainstage). A basic structure of the unit register circuit 60 in the k-thstage (1≦k≦N) is described with reference to the unit register circuit60 in a main stage (λth stage), and then, a structure of the unitregister circuit 60 in the dummy stages (k=1, 2, N−1, and N) isdescribed mainly with reference to points different from those of thebasic structure.

The unit register circuit 60 in the k-th stage includes NMOS transistorsT1F, T1B, T2, T4 to T6, T6A, T6B, T6C, T9F, and T9B and capacitors C1and C3.

The unit register circuit 60 in the k-th stage has an output terminalNOUT(k) for outputting a pulse G(2k−1) of its own stage. The unitregister circuit 60 in the k-th stage further has, as terminals to whicha pulse or a trigger signal in another stage is input, a forwarddirection set terminal NSF(k), a reverse direction set terminal NSB(k),a forward direction reset terminal NRF(k), and a reverse direction resetterminal NRB(k). An output signal G(2λ−3) is input to a terminal NSF(λ)in a main stage from a (λ−1)th stage, an output signal G(2λ+1) is inputto a terminal NSB(λ) from a (λ+1)th stage, an output signal G(2λ+3) isinput to a terminal NRF(λ) from a (λ+2)th stage, and an output signalG(2λ−5) is input to a terminal NRB(λ) from a (λ−2)th stage. Some of theinput terminals of the dummy stages do not have corresponding outputsignals in other stages. If so, a trigger signal is input to such aterminal. The dummy stages are to be described more specifically below.

Further, V(2k−1), V(2k+1), V(2k+3), and V(2k+5) are input to the unitregister circuit 60 in the k-th stage from the clock signal generatingportion 34. Further, to the unit register circuits 60, a voltage at theH level is supplied from a power supply VGH and a voltage at the L levelis supplied from a power supply VGL.

A drain of the output transistor T5 is connected to a signal line of theclock signal V(2k−1) and a source of the output transistor T5 isconnected to the output terminal NOUT(k), and the conduction of thetransistor T5 is controlled according to the potential of a referencepoint N1 connected to a gate of the transistor T5. The capacitor C1 isconnected between the gate and the source of the transistor T5. Thetransistor T5 and the capacitor C1 function as an output circuit whichoutputs the output pulse G(2k−1) of its own stage in synchronizationwith the clock pulse V(2k−1) with the reference point being at the Hlevel.

Drains of the transistors T6, T6A, T6B, and T6C are connected to theoutput terminal NOUT(k) and sources of the transistors T6, T6A, T6B, andT6C are connected to the power supply VGL. A gate of the transistor T6is connected to the node N2. A clock signal V(2k+1) is applied to a gateof the transistor T6A. The clock signal V(2k+3) is applied to a gate ofthe transistor T6B. A clock signal V(2k+5) is applied to a gate of thetransistor T6C. When the potential of the node N2, the clock signalV(2k+1), the clock signal V(2k+3), or the clock signal V(2k+5) is at theH level, the output terminal NOUT(k) is connected to the power supplyVGL.

The reference point N1 is connected to the terminals NSF(k) and NSB(k)via the diode-connected transistors T1F and T1B, respectively. Thetransistors T1F and T1B function as a set circuit which sets thereference point N1 at the H level when an output pulse of another stageis input to the terminal NSF(k) or NSB(k).

The transistors T2, T9F, and T9B which are connected between thereference point N1 and the power supply VGL so as to be in parallel toone another function as switch elements which providecontinuity/discontinuity between the reference point N1 and the powersupply VGL. A gate of the transistor T2 is connected to the node N2, agate of the transistor T9F is connected to the terminal NRF(k), and agate of the transistor T9B is connected to the terminal NRB(k). When thepotential of the node N2, the terminal NRF(k), or the terminal NRB(k) isat the H level, these transistors set the potential of the referencepoint N1 to the L level. In particular, the transistors T9F and T9Bfunction as a reset circuit which sets the reference point N1 to the Llevel when an output pulse of another stage is input to the terminalNRF(k) or NRB(k).

The transistor T4 is connected between the node N2 and the power supplyVGL. A gate of the transistor T4 is connected to the reference point N1.Further, the capacitor C3 is connected between the node N2 and an inputterminal of the clock signal V(2k−1). The transistor T4 functions as aswitch element which provides continuity/discontinuity between the nodeN2 and the power supply VGL. During a period in which the potential ofthe reference point N1 is at the H level, the transistor T4 is in the onstate, and sets the potential of the node N2 to the L level. On theother hand, during a period in which the potential of the referencepoint N1 is at the L level, the transistor T4 is in the off state. Inthis state, when the clock signal V(2k−1) is at the H level, thepotential of the node N2 is pulled up to the H level via the capacitorC3.

Next, the unit register circuit 60 in the dummy stages is described. Thecircuit structure of the unit register circuit 60 in the dummy stages iscommon to that in the main stages illustrated in FIG. 13. The dummystages are different from the main stages in that a signal from thetrigger signal generating portion 36 is input to some of the terminalsNSF, NSB, NRF, and NRB thereof. The different point is similar to thatin the first embodiment. More specifically, the forward directiontrigger signal VSTF is input to the terminal NSF in the first stage, andthe reverse direction trigger signal VSTB is input to the terminal NSBin the N-th stage. Further, the forward direction trigger signal VSTF,for example, is input to the terminal NRF in the (N−1)th and N-thstages, and the reverse direction trigger signal VSTB, for example, isinput to the terminal NRB in the first and second stages.

Further, as described in the first embodiment, it is preferred that thesize of the output transistors T5 in the dummy stages be made smallerthan that in the main stages.

In the above, the structure of the gate line driving circuits 14 isdescribed taking as an example the gate line driving circuit 14R fordriving the odd-numbered gate signal lines 20. The structure of the gateline driving circuit 14L for driving the even-numbered gate signal lines20 in this embodiment is similar to that of the gate line drivingcircuit 14R on the right side. In this regard, this embodiment issimilar to the first embodiment, and thus, the description thereof isomitted.

Next, operation of the bidirectional shift register 30 is described.Timing diagrams illustrating waveforms of various signals in the forwardshift operation and in the backward shift operation in this embodimentare the same as FIGS. 8 and 9, respectively, which are referred to inthe first embodiment.

The forward shift starts by, at the head of image signals for one frame,generation of a pulse of the forward direction trigger signal by thetrigger signal generating portion 36 (at the times t0 and t1 in FIG. 8).As described above, the clock signal generating portion 34 generates, inthe forward shift operation, pulses in sequence in the forwarddirection.

Here, first, forward shift operation of the unit register circuit 60 ina main stage (λth stage) in the gate line driving circuit 14R isdescribed.

Before the operation in the λth stage, the first to (λ−1)th stages areoperated in sequence to output pulses having a width of 2H with a phasedifference of 2H therebetween. When a pulse of the output signal G(2λ−3)in the (λ−1)th stage is input to the terminal NSF(λ) (time t2 in FIG.8), the reference point N1 is set to a potential (VGH-Vth (T1F))corresponding to the H level to turn on the transistor T5, and theinterterminal voltage of the capacitor C1 is set to the same potential.Here, the transistor T4 is turned on to set the node N2 to the L level.Therefore, the transistors T2 and T6 are in the off state.

At the time t3 after 2H from the time t2, a pulse of the output controlclock signal V(2λ−1) is input to the drain of the transistor T5. Thepulse of the clock signal V(2λ−1) pulls up the potential of the sourceof the transistor T5. Then, the potential of the reference point N1 isfurther pulled up by the bootstrap effect, and the pulse of the clocksignal V(2λ−1) becomes a pulse of the signal G(2λ−1) without loweringthe potential thereof to be output from the terminal NOUT(λ). The pulseof the signal G(2λ−1) is input to the terminal NSF in the (λ+1)th stage,and sets the reference point N1 in that stage to the H level.

When, at the time t4, the pulse of the output control clock signalV(2λ−1) falls, the pulse of the signal G(2λ−1) also falls. Further, atthis timing, a pulse of the clock signal V(2λ+1) turns on the transistorT6A, and thus, the output terminal NOUT(λ) is connected to the powersupply VGL and the output signal G(2λ−1) is at the L level. On the otherhand, the potential of the reference point N1 is held at the H level(subsequent set period).

At the time t4, the (λ+1)th stage outputs a pulse of the signal G(2λ+1)in synchronization with the pulse of the clock signal V(2λ+1). In thisway, each of the stages outputs a pulse of its own with a delay of 2Hfrom the output of a pulse in the preceding stage. The (λ+2)th stagewhich receives the output of the pulse in the (λ+1)th stage outputs apulse of the signal G(2λ+3) at a time t5 which is delayed by 2H from thetime t4.

In the λth stage, when the pulse of the signal G(2λ+3) is input to theterminal NRF at the time t5, the transistor T9F is turned on to resetthe reference point N1 to the L level. At the same time, a pulse of aclock signal V(2λ+3) turns on the transistor T6B to hold the outputsignal G(2λ−1) at the L level. It is to be noted that, in the subsequent2H period, the transistor T6C is turned on by a pulse of a clock signalV(2λ+5) to hold the output signal G(2λ−1) at the L level.

By the way, a pulse of the output control clock signal V(2k−1) isapplied to the drain of the transistor T5 even outside the set period ofthe reference point N1 (output period of the k-th stage), and the pulsehas a function to pull up the potential of the reference point N1 viathe gate-drain capacitance Cgd of the transistor T5. The potentialfluctuations at the reference point N1 are suppressed by turning on thetransistor T2. In a reset period of the reference point N1, thereference point N1 is basically at the L level and the transistor T4 isin the off state. In this state, as described above, the potential ofthe node N2 is pulled up to the H level according to a pulse of theoutput control clock signal V(2k−1), and the transistors T2 and T6 areturned on. This fixes the reference point N1 to the L level in the resetperiod, and the output signal G(2λ−1) is held at the L level.

As described above, a main stage receives an output pulse from theprevious stage to cause the reference point N1 to be in the set state,and receives an output pulse from the stage next to the subsequent stageto cause the reference point N1 to be in the reset state. However, thedummy stage as the first stage does not have the previous stage.Therefore, as described above, the first stage has a structure in whicha pulse of the forward direction trigger signal VSTF is input to theterminal NSF. The first stage receives a pulse of the signal VSTF whichis generated at the time t0 to set the reference point N1 to the Hlevel. Operation in the first stage after this is similar to that in theλth stage described above. Further, the (N−1)th and N-th stages whichare dummy stages do not have the stage next to the subsequent stage.Therefore, as described above, the (N−1)th and N-th stages have astructure in which a pulse of the forward direction trigger signal VSTFis input to the terminal NRF. The reference point N1 in the (N−1)th andN-th stages is set to the H level at the end of the forward shiftoperation for one frame, and is then reset to the L level according to apulse of the signal VSTF which is generated at the start of thesubsequent frame.

The forward shift operation of the stages in the gate line drivingcircuit 14R is described above. The forward shift operation of thestages in the gate line driving circuit 14L is similar to that of thecorresponding stages in the gate line driving circuit 14R. However, thestages in the gate line driving circuit 14L operate at a timing 1Hbehind the corresponding stages in the gate line driving circuit 14R,respectively.

The backward shift starts by, at the head of image signals for oneframe, generation of a pulse of the reverse direction trigger signal bythe trigger signal generating portion 36 (at the times t0 and t1 in FIG.9). As described above, the clock signal generating portion 34generates, in the backward shift operation, pulses in sequence in thereverse direction.

Similarly to the case of the unit register circuits 38 in the firstembodiment, in the unit register circuits 60 in the respective stages ofthe shift register portion 32, the set terminals and the reset terminalsare in a symmetrical relationship in structure with respect to theforward shift and the backward shift. Further, similarly to the firstembodiment, the top dummy stages and the bottom dummy stages are in asymmetrical relationship in structure with respect to shifts in oppositedirections. Therefore, if the control circuit 18 switches the triggersignals and switches the order of generation of the clock pulses, theshift register portion 32 performs the backward shift operationsimilarly to the forward shift operation.

For example, in the N-th stage in the gate line driving circuit 14R, apulse of the reverse direction trigger signal VSTB is input to theterminal NSB at the time t1 and the reference point N1 is set to the Hlevel. After that, in synchronization with a pulse of a clock signalV(2N−1) which is generated first, a pulse is generated in an outputsignal G(2N−1). In this way, pulses are output from the stages insequence in the opposite order to that in the forward shift operation.

The backward shift operation is described in the above taking as anexample the gate line driving circuit 14R. The backward shift operationof each of the stages in the gate line driving circuit 14L is similar tothat of the corresponding stage in the gate line driving circuit 14R.However, each of the stages in the gate line driving circuit 14Lperforms operation with an advance of 1H from the corresponding stage inthe gate line driving circuit 14R.

It is to be noted that various kinds of variations in structuredescribed in the first embodiment may also be adopted in thebidirectional shift register according to this embodiment.

While there have been described what are at present considered to becertain embodiments of the invention, it will be understood that variousmodifications may be made thereto, and it is intended that the appendedclaims cover all such modifications as fall within the true spirit andscope of the invention.

What is claimed is:
 1. A bidirectional shift register, comprising: ashift register portion including unit register circuits cascaded in mstages, where m is an integer which is equal to or larger than 3, foroutputting an output pulse P_(k) in a k-th stage, where k is an integerwhich satisfies 1≦k≦m, in a shift order which is one of a forwarddirection and a reverse direction; a clock signal generating portion forsupplying n-phase clock pulses to the respective stages of the shiftregister portion, where n is an integer which is equal to or larger than3, in sequence in the forward direction in forward shift operation ofthe shift register portion and in the reverse direction in backwardshift operation; and a trigger signal generating portion for generatinga forward direction trigger signal at the start of the forward shiftoperation and generating a reverse direction trigger signal at the startof the backward shift operation, the unit register circuit in a k-thstage including forward and reverse direction set terminals, forward andreverse direction reset terminals, a set circuit for setting a referencepoint of the unit register circuit to a first potential when a setsignal is input to one of the set terminals, a reset circuit for settingthe reference point to a second potential when a reset signal is inputto one of the reset terminals, and an output circuit for bootstrappingthe reference point and outputting the output pulse P_(k) from an outputterminal in synchronization with the n-phase clock pulse which is inputto the unit register circuit with the reference point of the unitregister circuit being at the first potential, where variables given asαf, αb, βf, and βb are natural numbers which satisfy both αf<βb<n andαb<βf<n, the set circuit in the k-th stage being input the output pulseP_(k−αf) when k>αf and the forward direction trigger signal when k≦αf tothe forward direction set terminal as the set signal and being input theoutput pulse P_(k+αb) when k≦m−αb and the reverse direction triggersignal when k>m−αb to the reverse direction set terminal as the setsignal, and the reset circuit in the k-th stage being input the outputpulse P_(k+βf), where k≦m−βf, to the forward direction reset terminal asthe reset signal after the output pulse P_(k) ends and being input theoutput pulse P_(k−βb) , where k>βb, to the reversed direction resetterminal as the reset signal after the output pulse P_(k) ends, theforward direction trigger singnal being input to the forward directionreset terminal in each of (m−βf +1)th to m-th stages as the resetsignal, and the reverse direction trigger signal being input to thereverse direction reset terminal in each of first to βb-th stages as thereset signal.
 2. The bidirectional shift register according to claim 1,wherein both αf and αb are
 1. 3. The bidirectional shift registeraccording to claim 1, wherein: the unit register circuit in each of(αf+1)th to m-th stages includes a circuit for, when the forwarddirection trigger signal is input, setting the reference point to thesecond potential; and the unit register circuit in each of first to(m−αb)th stages includes a circuit for, when the reverse directiontrigger signal is input, setting the reference point to the secondpotential.
 4. The bidirectional shift register according to claim 1,wherein: each of the unit register circuits includes a first transistorincluding a gate terminal that is connected to the forward directionreset terminal and a second transistor including a gate terminal that isconnected to the reverse direction reset terminal, each of the firsttransistor and the second transistor being provided between thereference point and a power supply of the second potential and, when thereset signal is applied to the gate terminal of each of the firsttransistor and the second transistor, being turned on to connect thepower supply to the reference point; and the trigger signal generatingportion applies a signal for turning on the second transistor to thereverse direction reset terminal in each of the first to βb-th stagesduring a period between repetitions of the forward shift operation, andapplies a signal for turning on the first transistor to the forwarddirection reset terminal in each of the (m−βf +1)th to m-th stagesduring a period between repetitions of the backward shift operation. 5.An image display device, comprising: a plurality of pixel circuitsarranged in matrix correspondingly to a plurality of scanning lines; aplurality of gate signal lines provided for the plurality of scanninglines, respectively, for supplying a gate signal for controlling writingof image data to the plurality of pixel circuits; and a gate signal linedriving circuit for, using the bidirectional shift register according toclaim 1, generating the gate signal to each of the plurality of gatesignal lines based on the output pulse which is output from a stagecorrelated with the gate signal line among the plurality of stages ofthe shift register portion.